Numerous applications, ranging from tactical and strategic defense systems (flight control, night vision, and early warning systems) to commercial technologies in public safety, industry, and healthcare require sensitive far-infrared microsensors to employ in high-density focal plane arrays. The high-temperature limitations of IR sensors are mainly conditioned by the tremendous decrease of sensitivity, caused by substantial reduction of photocarrier lifetime above liquid nitrogen temperatures. A problem for high-temperature operation of quantum well infrared photodetector (QWIP) detectors is the fast picosecond photoelectroncapture, which strongly limits the detector's responsivity and sensitivity due to significant generation-recombination noise.
One of the goals for next generation of imaging systems and solar cell photovoltaic devices is to increase the photoresponse to visible and infrared (IR) radiation. Improved electron coupling and increased carrier lifetime suppression of recombination can result in improved photoresponse. However, it is not easy to increase the radiation absorption without enhancement of recombination losses because by introducing electron levels that provide radiation-induced electron transitions inevitably creates additional channels for inverse processes that increase recombination losses.
This trade-off between absorption and recombination processes are well understood for a number of technologies and corresponding materials. For example, starting from the early sixties significant attention was attracted to semiconductors with impurities, which provide electron levels inside the semiconductor bandgap and in this way induce the IR transitions from localized impurity states to conducting states in the semiconductor material. However, midgap impurities drastically enhance the recombination processes, i.e., Shockley-Read-Hall recombination, and deteriorate the photovoltaic conversion efficiency.
To accommodate the solar spectrum and to utilize its IR portion, the modern photovoltaic technology mainly employs multi-junction cells with different electron bandgaps. In these devices each p-n junction cell is designed to effectively harvesting solar energy within a certain spectral window close to the bandgap. According to the theoretical modeling, in a multi-junction solar cell with five or more junctions the ultimate photovoltaic efficiency may exceed 70%. However, current technology enables to produce only triple-junction cells (Ge-substrate junction-InGaAs—AlInGaP) with the maximum conversion efficiency of ˜40% for concentrator cells. Strong technological limitations are caused by the need for lattice match, thermal expansion match, and current match in the cascade of heterojunctions.
Quantum-dot structures are considered for use in photovoltaic nanomaterials due to their ability to extend the conversion of the solar energy into the infrared range. Up to now the most efforts were concentrated on the quantum-dot solar cell with intermediate band structure, which is formed from discrete QD levels due to tunneling coupling between QDs. Theoretical calculations predict that the intermediate band solar cell can provide efficiency of ˜65%.
However, intensive experimental efforts to improve performance of the intermediate band solar cells show limited success. In comparison with a reference cell, the photovoltaic efficiency of the QD intermediate band cells increases just by 1-2% percent. It is well understood that addition of QDs significantly increases the absorption of IR radiation, but simultaneously QDs drastically increase recombination processes. For this reason, the corresponding recombination losses are hardly compensated by the conversion of IR radiation.